You can set a new user agent inside of a running template by using Profile Processing > Reassign Field. Set Parameter to 'userAgent', Action: 'Set', Value: your new user agent.
But take into consideration that wen new Zenno version restarts, it does much more than setting a new user agent. It emulates screen resolutions according to the chosen operating system, it handles canvas, webgl etc...and these things are a bit more difficult to adjust manually. Personally, I would develop a solution in which the instance is restarted and let Zenno pick the best user agent settings combination.
Always clear cache and cookies if you want your template to emulate a new/ different user.